Hypogonadotrophic hypogonadism in type 2 diabetes

Abstract

Recent work shows a high prevalence of low testosterone and inappropriately low luteinizing hormone (LH) and follicle stimulating hormone (FSH) concentrations in type 2 diabetes. This syndrome of hypogonadotrophic hypogonadism (HH) is associated with obesity in patients with type 2 diabetes. However, the duration of diabetes or HbA1c are not related to HH. Furthermore, recent data show that HH is not associated with type 1 diabetes. C-reactive protein concentrations have been shown to be elevated in patients with HH and are inversely related to plasma testosterone concentrations. This inverse relationship between plasma free testosterone and C- reactive protein concentrations in patients with type 2 diabetes suggests that inflammation may play an important role in the pathogenesis of this syndrome. This is of interest since inflammatory mechanisms may have a cardinal role in the pathogenesis of insulin resistance. It is also relevant that in the mouse, deletion of the insulin receptor in neurons leads to HH in addition to a state of systemic insulin resistance. It has also been shown that insulin facilitates the secretion of gonadotrophin releasing hormone (GnRH) from neuronal cell cultures. Thus, HH may be the result of insulin resistance at the level of the GnRH secreting neuron. Low testosterone concentrations are also related to an increase in total and regional adiposity. This review discusses these issues and attempts to make the syndrome relevant as a clinical entity. Clinical trials are required to determine whether testosterone replacement alleviates insulin resistance and inflammation. In addition, low testosterone levels are associated with an increase in cardiovascular events. Testosterone therapy may therefore, reduce cardiovascular risk. This important aspect requires further investigation.

Keywords:

• Hypogonadism
• diabetes
• obesity
• testosterone
• insulin resistance
• hypogonadotrophic
• metabolic syndrome

Introduction

Large epidemiological studies have demonstrated the association of low testosterone (T) with type 2 diabetes (T2D) [1,2]. In these studies T was treated as a biological marker which was found to be frequently low in males with T2D, often in association with dyslipidaemia and abdominal adiposity [1]. It was also shown that insulin resistance and upper abdominal adiposity were features associated with low T [3,4]. These studies did not report sex hormone binding globulin (SHBG), free T (FT) or gonadotrophin concentrations and thus could not make a comment on the mechanism underlying low T concentrations, nor did they place low T concentrations in a clinically relevant context.

In view of the above and our own recent work demonstrating the frequent occurrence of hypogonadotrophic hypogonadism (HH) in men with T2D, the inverse relationship between BMI and low T [5] and the marked increase of C-reactive protein (CRP) in HH [6], there is potentially an important role for a low T in T2D. In addition, type 1 diabetes(T1D) is not commonly associated with HH [7]. Thus, clearly HH is a feature related to obesity, insulin resistance and T2D rather than hyperglycaemia per se. This growing field in endocrinology needs an organized systematic investigation including prospective controlled studies on the treatment of HH in T2D to determine whether clinical features associated with hypogonadism can be successfully reversed with T replacement. A review of the currently available literature in this area is thus clearly required.

The cross-sectional association of hypogonadism with type 2 diabetes

Association with type 2 diabetes

T concentrations in men have been found to be inversely related to insulin, independently of age and obesity [8]. Several investigators have studied the association of low total T with diabetes [1,2],[9,10],[15-19]. In a population-based cohort study of 985 men (40–79 years), 110 men with T2D had lower mean T and SHBG concentrations [2]. These differences were reduced but not eliminated after adjustment for age and BMI. Of diabetic men, 21% versus 13% of controls had a low T concentration. T was inversely related to the degree of glycaemia in the whole cohort; throughout the whole range of plasma glucose, there was a stepwise decrease in mean T per categorical increase in FPG, apparent in both diabetics and non-diabetics. Anderson et al. also found low T and SHBG concentrations in male T2D [10]. T concentrations were inversely related to fasting insulin concentrations. In a community study, comprising 1,408 older subjects (775 men) with mean age of 73 years, Goodman et al. found that in sex-specific age and BMI adjusted analyses, men with impaired glucose tolerance had significantly lower total T than those with normal glucose tolerance and that T was inversely related to fasting plasma glucose [11]. A recent study in 1,200 men with erectile dysfunction found low total T in 24.5% of T2D as compared to 12.6% in non-diabetic patients; differences in the prevalence of hypogonadism retained significance after adjustment for age and BMI [12].

All these studies measured total T and SHBG but not FT because reliable assays for FT have become available only recently. SHBG concentrations are decreased in obesity. T2D men have even lower SHBG levels as compared to age and BMI matched non-diabetics [2]. It was therefore not clear whether the lower SHBG levels in T2D could account for the difference observed in total T levels between diabetics and non-diabetics.

Total T concentrations are determined to a large extent by circulating SHBG concentrations. In the blood of normal men, 44% of total T is bound to SHBG, 2% is unbound (FT) and 54% circulates bound to albumin and other proteins [13]. Since albumin bound T has 1,000 times lower affinity than SHBG, it can freely disassociate in capillaries. Virtually all the non-SHBG bound T (also called bioavailable T) is therefore available for tissue uptake [14]. Circulating SHBG concentrations are also dependent upon a number of factors, the most important association being with obesity. SHBG concentrations decrease in obesity and increase with aging. A complete assessment of hypogonadism should therefore include measurement of FT. FT measured by radioimmunoassay is considered unreliable because it represents a variable fraction (20% to 60%) of the FT measured by equilibrium dialysis [15-17]. Equilibrium dialysis is considered to be the gold standard for measuring FT. FT measured by this technique represents 1.5% to 4% of total T and is not dependent upon SHBG concentrations [18]. Equilibrium dialysis is a delicate, tedious and time-consuming technique and therefore may not suitable for population-based or large-sized studies. FT can also be calculated from SHBG and T using the method of Vermeulen et al. [16]. This calculated FT has been shown to correlate very well with FT measured by equilibrium dialysis [15] and is well suited for epidemiological studies.

The first study to report the prevalence of hypogonadism in T2D based on appropriately FT measured by equilibrium dialysis was published in 2004 [5]. This study included 103 males with T2D, aged between 31 and 75 years, referred to a tertiary diabetes centre in Buffalo, NY. The prevalence of hypogonadism in T2D men was 33%. The plasma concentrations of LH and FSH were inappropriately low in men with subnormal FT. Indeed, LH and FSH were significantly lower than those in patients with normal FT. In addition, GnRH induced LH and FSH release was normal in all those who were tested. This pointed to a hypothalamo-hypophyseal lesion rather than a testicular defect. Magnetic resonance imaging (MRI) in 10 randomly selected hypogonadal patients revealed no abnormality in brain or the pituitary. There was an inverse relationship of total T and FT with BMI. Thus, the more obese the patient, the lower were total T and FT. HH was also reported in male T2D with erectile dysfunction [19]. Another study has also found high prevalence of hypogonadism in male T2D (50% on that basis of calculated FT concentrations) [20]. Forty-two per cent of T2D patients had symptoms of hypogonadism along with low FT concentrations. BMI and waist circumference were negatively related to total and bioavailable T. Three quarters of the men with hypogonadism had low or inappropriately low gonadotrophin concentrations. Pituitary MRI did not reveal any abnormalities in men with low gonadotrophins [20]. A recently concluded multicentre study (Hypogonadism in Males (HIM)), designed to determine the prevalence of hypogonadism in males over the age of 45 years (mean age: 62 years) in the United States, found the prevalence of hypogonadism to be 38.4%[21]. The study defined hypogonadism as total T < 300 ng/dL. The prevalence in male diabetics (over the age of 45 years) was 50%. Although no distinction was made between T1D and T2D diabetics, it is reasonable to conclude that the overwhelming majority of these patients had T2D. The patients with a low T had a greater prevalence of hypertension, hyperlipidaemia, obesity, inadequate sexual function and libido than patients with normal gonadal function.

What comes first? Hypogonadism or T2D?

Review of the literature clearly shows that hypogonadism is associated with T2D in a substantial number of men. Obesity might be the most important contributor to the presence of hypogonadism in T2D, and the linear inverse relationship of T with BMI is preserved in the presence of T2D [8],[10]. The relationship of T2D and obesity with low T is probably bidirectional. Low T can predispose to obesity while obesity perpetuates hypogonadism. But what comes first? Several epidemiological studies have looked at the predictive value of hypogonadism on the development of T2D over a number of years in men. Studies from Barrett-Conner et al. have shown that low T in men and high T in women predict insulin resistance (IR) and T2D in older adults [22]. A case control study of 176 non-diabetic men followed up for 5 years showed that low levels of SHBG and T at baseline were associated with the development of T2D in men [23]. A recent analysis from the NHANES III [24] showed that low androgens were a risk factor for diabetes in men. In multivariable models adjusted for age, race/ethnicity, and adiposity, men in the lowest tertile of FT concentrations were four times more likely to have diabetes compared with men in the third tertile (odds ratio 4.12 [95% CI 1.25–13.55]). These associations persisted even after excluding men with low total or FT concentrations (suggesting that the risk was not completely driven by hypogonadal men).

The high prevalence of hypogonadism in T2D raises several questions: first and foremost, does this defect also occur in T1D? We compared the prevalence of hypogonadism in age matched (mean age 43 years) T1D and T2D men. Six per cent of T1D mean were hypogonadal as compared to 26% of T2D men [7]. SHBG in T1D was higher than that in T2D men. Thus, HH does not occur commonly in T1D, and so is not a function of diabetes or hyperglycaemia per se. Interestingly, however, TT and FT are inversely related to BMI even in T1D [7]. The next question that arises is whether HH is related to insulin resistance, especially since there is an inverse relationship of TT and FT with BMI in both T1D and T2D (Figure 1, correlation of calculated free testosterone with BMI in type 1 and type 2 diabetic subjects). In addition, previous studies have shown that hypogonadism is associated with upper abdominal adiposity, hyperinsulinaemia and insulin resistance [1],[3,4],[10],[22],[25-33]. Thus, there is prima facie evidence that HH may be related to insulin resistance, and perhaps hyperglycaemia in addition to insulin resistance makes the defect more frequent and possibly more severe. With a prevalence of approximately one third in T2D, HH is a potential candidate for being the commonest complication in males with T2D (through association rather than causality at this time) and requires further assessment in terms of the etiology of the defect and the possible consequences, complications and treatment.

Figure 1. Correlation of calculated free testosterone (nmol/l) with BMI (kg/m²) in age matched type 1 (○; r = –0.36, P < 0.05) and type 2 (•; r = –0.42, P < 0.05) diabetic subjects.

Hypogonadism and insulin sensitivity

Relationship between hypogonadism and insulin sensitivity

Insulin sensitivity is inversely related to intramuscular, intramyocellular and intra-abdominal adipose tissue. Hypogonadism is also associated with increased subcutaneous (both truncal and appendicular), intra-abdominal and intramuscular adipose tissue [34,35]. Epidemiological studies clearly show an inverse relationship between T and insulin resistance, probably mediated by total or abdominal adiposity [36]. Insulin resistance and visceral obesity is associated with low SHBG and low total T levels in men [1],[3,4],[10],[22],[25-33],[37,38]. A study on patients with Klinefelter's syndrome found decreased insulin sensitivity which correlated with truncal obesity [39]. Serum LDL-C, triglycerides and CRP concentrations were also markedly increased. These abnormalities were reversed partially after replacement with T, although the doses of T given for replacement were small. We have recently evaluated the relationship of FT concentrations with body composition as measured by DEXA in men with T2D [40]. FT concentrations were negatively related to all measures of fat mass (appendicular, trunk and total fat mass) and positively related to arm lean mass. The relationship between insulin resistance and hypogonadism may be bidirectional: insulin resistance may promote hypogonadotrophism and hypogonadism may promote obesity and insulin resistance [41,42] Decrease in insulin resistance by rosiglitazone leads to an increase in T concentrations in T2D men [43].

Interestingly, the effect of testosterone on insulin sensitivity may also be mediated by mechanisms other than change in adiposity. Pitteloud et al. looked at the relationship between testosterone levels, insulin sensitivity, and mitochondrial function in 60 men and found that T correlated with VO₂ max and oxidative phosphorylation gene expression independent of BMI [38]. A recent trial studied the effect of withdrawal of T therapy on insulin resistance in 12 men with idiopathic HH who were on chronic replacement with T [44]. Stopping T therapy for only two weeks worsened insulin sensitivity by 40% compared to baseline. Insulin sensitivity was measured by HOMA-IR in the study. This was also associated with an increase in serum IL-6 concentrations but a decrease in TNF-α.

Studies in androgen receptor knock out (ARKO) mice show that male ARKO mice develop obesity while female ARKO mice do not [45]. The obesity in male ARKO mice is not due to hyperphagia but due to decreased spontaneous activity and decreased overall oxygen consumption [46]. Interestingly, these mice had preserved insulin sensitivity, possibly due to elevated adiponectin secretion. Serum estrogen concentrations were normal in ARKO mice. A mouse model of estrogen deficiency, aromatase deficient (ARKO) mouse, also demonstrates obesity and hepatic steatosis in males but not in females [47].

Effect of testosterone replacement on insulin sensitivity

T replacement leads to a dose-dependent decrease in adipose tissue and an increase in muscle mass and strength [34],[48,49]. This effect appears to be more pronounced if the population studied has higher amounts of adipose tissue. The mechanism by which T replacement produces these effects is not well understood. Inhibition of lipoprotein lipase activity in intra-abdominal adipose tissue and the differentiation of pluripotent mesenchymal precursor cells preferentially into myogenic lineage instead of adipocytic lineage may play a role [50,51]. Marin et al. demonstrated a decrease in visceral adipose tissue and an improvement in insulin sensitivity with oral and transdermal T treatment in middle-aged obese non-diabetic men [52,53]. The subjects were not hypogonadal but insulin sensitivity improved more in subjects whose T concentrations were in the low normal range at the start of the study. The improvement in insulin sensitivity and basal plasma T concentrations were inversely related. In one of these studies, oral T undecanoate 80 mg twice a day (which increased serum T to 500–600 ng/dL for a few hours after the dosing) or placebo given for 8 months reduced visceral body fat (measured by CT scan) and total body fat by approximately 6%. Subcutaneous adipose tissue did not change. Glucose disposal rate (measured by hyperinsulinaemic-euglycaemic clamp) increased by 20%. Change in glucose disposal rate was related to baseline T levels (r = −0.65, P < 0.05). In another publication [53] using a similar population, transdermal T for 3 months (which doubled T from 418 ng/dL at baseline to 820 ng/dL) increased glucose uptake in hyperinsulinaemic-euglycaemic clamp by 17% and decreased waist/hip circumference ratio. However, studies done in non-obese men have failed to show an improvement in insulin sensitivity [54-56]. The effect of T on adipose tissue, muscle mass and insulin sensitivity seems to depend on the population studied, the dose used for T replacement and the duration of therapy. It is possible that the improvement in insulin sensitivity after T replacement is due to an increase in lean body mass and a decrease in adipose tissue mass, or more specifically by a decrease in visceral or intramyocellular adipose tissue, and that this effect is more pronounced in the hypogonadal patients.

No large studies have been done to evaluate the effect of T on insulin sensitivity in T2D patients. A small study in 24 hypogonadal type 2 diabetics showed improvement in Homeostasis Model Assessment (HOMA)-IR, HgbA1c and waist-hip ratio after treatment with intramuscular T for 3 months [57]. The effects of T replacement in T2D subjects, a population that is likely to benefit from a reduction in adipose tissue and an increase in muscle mass, would be of great interest. It clearly needs to be done in view of the frequency of HH in T2D.

Pathophysiological mechanisms underlying hypogonadotrophic hypogonadism in insulin resistance

Insulin resistance and hypogonadotrophic hypogonadism

Animal data

The best insight into the mechanism underlying HH with a probable defect in GnRH secretion from the hypothalamus is obtained from the mouse with an insulin receptor deletion in neurons (NIRKO), as described by Bruning et al. [58]. The selective deletion of the insulin receptor from neurons led to HH leading to a reduction in LH concentrations by 60%–90%. These animals responded to GnRH challenge by normal or supra normal release of LH. These data imply that insulin action in hypothalamic neurons may facilitate GnRH and thus gonadotrophin secretion. These mice also had atrophic seminiferous tubules and absence of mature spermatozoa in these tubules. In female NIRKO mice, interestingly, there was an absence of follicles in the ovary while the ovaries were smaller and had more fibrous tissue. Mice with IRS-2 deletion (in all tissues) are known to develop T2D and are profoundly insulin resistant and atherogenic. In addition, they are obese, have impaired pituitary development, have low gonadotrophin and have small, anovulatory ovaries with a reduced number of follicles [59]. Insulin has other actions on the hypothalamus: to mediate suppression of appetite through an action on the arcuate nucleus and to suppress hepatic glucose production mediated through the vagus. Furthermore, it is known that inflammatory mediators, inflammatory cytokines, tumour necrosis factor(TNF)-α and interleukin(IL)- 1β, have been shown to reduce hypothalamic GnRH and LH secretion in animals and in vitro[60,61].

Human studies

In 1993, Vermeulen et al. found attenuated luteinizing hormone (LH) pulse amplitude but normal LH pulse frequency in hypogonadal obese men, suggesting a likely hypothalamic GnRH or a pituitary secretory defect [62]. In this study, LH pulsatility over 12 hours was compared in obese and lean men. The mean integrated LH levels over 12 hours were significantly lower in obese men. FT concentrations correlated positively with the sum of LH pulse amplitudes in each individual. Further work from the same group showed low FT, LH and LH pulse amplitude in severely obese (BMI > 40kg/m²) while these parameters were normal in milder obesity [63]. One study suggested that obesity can also blunt the responsiveness of pituitary gonadotrophin secretion to GnRH stimulation [64]. Other studies showed a rise in LH or FSH levels after weight loss [65,66].

It should also be noted that LH/hCG-induced T secretion by Leydig cells is inversely correlated with insulin sensitivity (as measured by hyperinsulinaemic-euglycaemic clamp) among men with varying degrees of glucose tolerance [37]. Insulin receptors are present on Leydig cells [67]. Thus, the lesion resulting in hypogonadism in obesity and T2D may occur at multiple levels of hypothalamic-pituitary-gonadal axis [48-50]. However, the absence of hypergonadotrophinism indicates that the primary defect in T2D is at the hypothalamo-hypophyseal level.

Interestingly, the decline in T with aging appears to be associated with similar mechanisms. Mulligan et al. [68] in 1999 performed a randomized double blinded placebo controlled study in which a 2-week pulsatile GnRH infusion unmasked a dual (hypothalamic and Leydig cell) defect in the healthy aging male. This study did not include obese or diabetic patients.

Role of estradiol

Elevated estradiol and leptin concentrations observed in obesity probably mediate a part of the hypogonadotrophic effect of obesity, but they do not account for the full effect of obesity on hypothalamo-hypophyseal axis [66],[69,70]. Testosterone and androstenedione in the male can be converted to estrogen (estradiol and estrone respectively) via aromatization in extraglandular tissues, most significantly in adipose tissue. The rate of extraglandular aromatization increases with age and obesity. Even though aromatase activity increases with age, estrogen concentrations fall with age in males [71]. This is due to the fact that testosterone decreases with age and therefore is not available for conversion to estrogen.

Both testosterone and estradiol can act independently on pituitary/hypothalamus to induce negative feedback. Estradiol levels are increased in mild to moderate obesity [62,63]. FT continues to fall as the obesity progresses. Treating obese men with an aromatase inhibitor leads to a reduction in serum estrone and estradiol, and an increase in serum testosterone and LH/FSH levels [72]. The increase in FT is sometimes supraphysiological. Weight loss leads to an increase in testosterone and FSH concentrations; however serum estrogen levels either decrease or do not change with weight loss [66],[69]. This discrepancy may be related to the final weight achieved. It appears that even mild to moderate obesity can raise serum estrogen levels and achievement of normal weight normalizes serum estrogen levels. On the other hand, massively obese males may not have a significant change in their serum estrogen levels after weight loss if they are still obese at the end of a weight loss study, even though they may have an improvement in their FT levels.

It is not yet known if hypogonadal T2D males have higher or lower estrogen concentrations than eugonadal T2D males.

Role of leptin

Obesity is associated with increased plasma levels of leptin. Isidori et al. studied the relationship of leptin with sex hormones (one fasting blood sample) at varying degrees of obesity, normal (BMI < 30, n = 10) moderate obese (BMI 30–40, n = 14) and massive obese (BMI > 40, n = 14) [70]. Moderately obese subjects had higher leptin than non-obese subjects but did not differ in terms of most sex hormones (FT was lower but was measured by RIA and therefore unreliable). Massively obese subjects had higher leptin and estradiol levels and lower TT, FT, and SHBG levels (as compared to normal subjects). LH/FSH concentrations were not different between the groups. In multiple regression analysis of T with estradiol, leptin, SHBG and LH, leptin was the best predictor of fasting T concentrations. When all the subjects were stimulated with a single dose of human chorionic gonadotropin (hCG), obese subjects had a lower peak T concentration than non-obese subjects. Another study also found a negative correlation between leptin concentrations and hCG stimulated T levels [37]. Leptin receptors are present on Leydig cells [73]. This implies that leptin may play a role in decreased Leydig cell responsiveness to gonadotrophin stimulation in obesity.

Leptin is known to play a permissive role in the regulation of reproductive axis. Absence of leptin results in HH [74]. Leptin appears to serve as a signal of energy reserves to regulate the hypothalamo-pituitary-gonadal axis in relation to nutritional status [75]. It is not yet clear what role leptin resistance might play in the HH associated with insulin resistance.

Role of inflammation

Both obesity and diabetes have been shown to be associated with oxidative stress [76-79] and inflammation [80-84]. The concentration of pro-inflammatory cytokines including TNFα is elevated in T2D [82] and may contribute to HH in this condition. Obesity is also associated with an increase in pro-inflammatory cytokines like TNFα[81],[85,86]. This is consistent with the increase in hypogonadism in obesity and may also explain the effect of BMI on T concentrations in T2D and T1D.

The understanding of mechanisms leading to oxidative stress and inflammation is important. One possible reason why obesity and T2D are associated with oxidative stress and inflammation is the state of insulin resistance. This is due to the fact that 1) insulin resistance is associated with the presence of pro-inflammatory factors including cytokines [87]. These cytokines also interfere with insulin signal transduction [87] and this may also interfere with GnRH secretion since insulin is known to facilitate GnRH secretion both in vivo and in vitro[88]; 2) insulin has been shown to exert an anti-inflammatory and anti-oxidant effects at the cellular and molecular level both in vitro, and in vivo[89-92]. It is therefore relevant that insulin infusion induces an increase in plasma T [93].

Inflammatory mediators may impair insulin mediated mechanisms which facilitate GnRH, gonadotrophin and T secretion. This impairment is likely to be at the hypothalamic level in view of the defect described above. TNFα and IL-6 are known to reduce insulin receptor tyrosine phosphorylation, and IRS tyrosine phosphorylation and the phosphorylation of AkT kinase (protein kinase B). These effects have been shown in adipocytes, human aortic endothelial cells and hepatocytes [94]. TNFα has also been shown to reduce insulin receptor expression in human aortic endothelial cells [67]. It is probable that insulin signal transduction related defects would also be observed in neurons. More recently, it has been shown that SOCS-3, a protein which suppresses insulin signal transduction at IRS-1 level is induced by TNFα and IL-6 [95,96]. It is elevated in the obese and thus may have a role in the pathogenesis of a putative insulin resistant state at the level of GnRH neuron [97].

It has been suggested that hypogonadism is associated with an increase in inflammatory mediators and treatment of hypogonadism leads to a reduction in inflammation [98]. In vitro studies have demonstrated that T induces an inhibition of IL-6 production by human monocytes [99]. T treatment of human aortic endothelial cells results in an inhibition of TNF-α induced vascular cell adhesion molecule (VCAM)- 1 mRNA expression and nuclear factor-κB (a key inflammatory mediator) activation [100] Intramuscular T replacement in hypogonadal males results in a decrease in pro-inflammatory cytokines (IL-1 and TNF-α) and an increase in anti-inflammatory cytokine (IL-10) levels in serum [101]. However, some studies were unable to demonstrate an anti-inflammatory effect of T [54],[102]. It is possible that the effects of T replacement on inflammation depend on the population studied. A comprehensive study on inflammation and T replacement needs to be done to clarify the issue.

Inflammation is known to be a cardinal causative factor in inducing atherosclerosis [103,104]. Adherence of circulating monocytes and lymphocytes to the arterial endothelial lining is one of the earliest detectable events in human and experimental atherosclerosis. Is this adherence of monocytes reduced by T?

Type 2 diabetes, hypogonadism and atherogenesis

Epidemiological studies have shown that hypogonadism is associated with increased incidence of atherosclerosis [105]. Patients with coronary artery disease (CAD) have lower testosterone levels than healthy controls, and this is inversely related to the degree of CAD [106]. Hak et al. studied abdominal aortic calcifications (detected radiographically) in 504 non-smoking men in a population-based Rotterdam study [107]. They found an inverse relationship between T and abdominal calcifications. The age-adjusted relative risk of having abdominal calcifications was only 0.4 in men in the highest tertile of T and BT as compared to men in the lowest tertile. A population-based prospective study over 16 years [108] found that the ratio of cortisol to testosterone in men was directly related to incidence of ischaemic heart disease and to death due to ischaemic heart disease. These relationships were not significant when adjusted for HOMA-IR, BMI and triglycerides. Thus it may appear that there is an inverse relation of T with atherosclerosis, part of which may be explained by obesity. A recent population-based prospective study in elderly men found that men in the lower quartile of testosterone ( < 241 ng/dl) had 1.4 times higher risk of mortality (and 1.38 times higher risk of cardiovascular mortality) as compared to men in the highest quartile (total T > 370 ng/dl). This association was independent of adiposity but was attenuated by CRP and IL-6 [109]. The results were similar with bioavailable testosterone.

In a Finnish study of middle-aged men, carotid IMT was found to correspond inversely with serum T independent of age, cholesterol levels, smoking, blood pressure and BMI [110]. One study which measured total T, SHBG and calculated FT and IMT in men with mean age 60.3 years and BMI 26.1 kg/m² found no relation of calculated FT with IMT [111]. It found an inverse relation of total T and SHBG with IMT, but these relationships were not independent of BMI. However Makinen et al. [110] found an inverse relation between T and carotid IMT independent of BMI. They did not measure or calculate FT. There have been no studies that have quantitatively demonstrated the differences in carotid IMT within hypogonadal and eugonadal T2D patients.

In this regard, it is interesting that T has been shown to have anti-anginal properties in humans [112-114]. Animal studies have shown that high physiological doses of androgens (T or DHEA) decrease aortic atherosclerosis in cholesterol-fed castrated rabbits [115]. Testosterone administration in the orchidectomized LDL-receptor deficient mouse reduces and delays atherosclerosis. Testosterone also induces VCAM-1 expression in endothelial cells. Thus, testosterone may have an anti-inflammatory and anti-atherogenic function [116,117]. It is possible that treatment of hypogonadism can lead to decreased atherosclerosis in humans, but long-term studies looking at this effect need to be carried out.

It is therefore relevant that a recent study in T2D men has found markedly increased CRP concentrations in hypogonadal T2D as compared to T2D with normal T(6.5 mg/dl versus 3.2 mg/dl) [6]. The values of CRP observed in HH would place them in a group with the highest cardiovascular risk. There was an inverse relationship between CRP and calculated FT (CRP versus FT: r = −0.27; p = 0.02) (Figure 2, the inverse relationship between FT and CRP). The relationship of FT with CRP concentration was independent of BMI.

Figure 2. The inverse relationship between calculated FT and CRP (r = −0.27; p = 0.02).

It is therefore possible that a low T contributes to the overall inflammatory state of a patient with T2D. Another possibility is that the pro-inflammatory state, of which an elevated CRP concentration is an indicator, may lead to the suppression of GnRH secretion by the hypothalamic neurons and thus lead to HH. HH, which is also associated with greater BMI, may contribute to increased inflammation and atherogenesis in the long term in T2D. This group of HH males with T2D, hence, may be more vulnerable to cardiovascular events. A trial of T replacement in T2D male patients with HH is necessary to determine whether it reverses the increase in CRP and retards atherosclerosis.

Conclusion

The states of obesity and T2D, but not T1D, are clearly associated with frequent hypogonadism. In view of the increased risk of HH with an increase in BMI and the known association of hypogonadism with obesity, insulin resistance probably contributes to the pathogenesis of HH. Hyperglycaemia with insulin resistance may worsen the pro-inflammatory state, insulin resistance and HH. While the pathological defect in the secretion of T may be at several different levels, it is clear that the major/dominant defect is at the hypothalamic level since the concentrations of FSH and LH are inappropriately low and the response of these hormones to GnRH is normal. The mechanism underlying this defect is not entirely clear but insulin resistance and the associated inflammatory mediators may contribute to this defect since 1) NIRKO mice exhibit this defect and 2) insulin facilitates GnRH secretion by the GnRH secreting neurons.

Plasma CRP concentrations in HH population with T2D are markedly elevated and point to a heightened risk of cardiovascular disease. Furthermore, the increase in adiposity in this population in parallel with a reduction in FT and a simultaneous reduction in muscle mass may contribute to insulin resistance and a pro-inflammatory state. Clearly, trials of treatment with T need to be carried out to a) determine the reversibility of features related to HH and b) determine the definitive pathogenic role of T in these conditions. T replacement therapy offers opportunities in this patient population in terms of reduction of upper abdominal adiposity and an increase in muscle mass and insulin sensitivity.

Future directions

As detailed above, hypogonadism could be one of the mechanisms contributing to increased total and regional adiposity, reduced total and regional lean body mass, reduction in insulin sensitivity and increased oxidative and inflammatory stress in these subjects. It could also contribute to the higher incidence of cardiovascular disease in this population [118,119]. Also, as the youngest T2D patient with HH in one study [5] was 31 years old, HH could have an impact on the fertility of these patients. It is therefore extremely important that we define the effect of HH and the pathogenic mechanisms underlying it. The potential beneficial effects of the replacement of testosterone on the above parameters in T2D males need to be defined by undertaking appropriate prospective trials. It would also be important to evaluate the quality of semen and spermatozoal function. Testosterone therapy is not likely to improve this, and so gonadotrophin treatment may be required. Fertility would be more of an issue in younger patients in whom a focused study must be carried out separately. In view of the possible implications of low T in the pathogenesis of atherosclerosis and inflammation, studies directed towards these issues are also of paramount importance. The results of such trials will have a significant impact on the effect and treatment of HH in males with T2D. The findings will provide a clear rationale for future long term outcome trials of T therapy in T2D with HH.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.